Depending on the provenance, crude oils contain greater or lesser fractions of dissolved n-paraffins which represent a special group of problems, because they crystallize out on lowering of the temperature and thus can lead to a deterioration in the flow properties of these oils. In pipeline transport, this crystallization can cause depositions on the wall, and in an extreme case, complete blockage of the pipeline. In addition, paraffin precipitations can also lead to complications in production, storage and further processing.
The deposition of paraffin starts with the formation of microcrystals in the form of fine platelets and needles, and the viscosity of the crude oil rises significantly. When these microcrystals grow together into a three-dimensional network, the crude oil loses its fluidity and finally solidifies. The solidification point is also called setting point (pour point) and depends on the quantity and distribution of the paraffins dissolved in the oil.
For restoring or maintaining the fluidity, there are a number of measures of a thermal or mechanical nature--for example scraping the paraffin off the inner wall of the pipe, or heating of entire pipelines. It is certainly more elegant to combat the cause of the phenomenon by the addition of so-called setting point depressants (paraffin inhibitors, pour point depressants), in most cases in quantities of a few hundred ppm.
The effect of a paraffin inhibitor is based on the fact that it co-crystallizes with the paraffins, i.e. interferes with the build-up of the regular crystal structure. At the same time, the high-molecular inhibitor molecules represent crystallization nuclei, so that the number of primarily formed microcrystals is increased. Finally, polar groups within the inhibitor molecule prevent the crystals from growing together into a network. The paraffin inhibitor thus does not change the quantity of the paraffin crystallizing out, but the size and polarity of the crystals. The consequence is that the pour point is depressed and the fluidity of the oil is maintained across a wider temperature range.
The paraffin inhibitors are in general homopolymers and copolymers based on olefins, (meth)acrylate esters, maleic acid derivatives and certain vinyl monomers. Thus, for example, homopolymers of acrylates having 18 to 24 carbon atoms in the alcohol radical are claimed in German Offenlegungsschrift 2,264,328 for the use as pour point depressants.
In European Patent 0,120,512, copolymers of long-chain acrylate esters and comonomers such as styrene, t-butylstyrene, acrylonitrile, acrylamide or vinylpyridine are described for the same intended use.
Finally, U.S. Pat. No. 4,547,202 claims copolymers of long-chain acrylate esters and certain vinylidene components for crude oils and residual oils.
A disadvantage of all the substances mentioned is that their activity is not yet sufficient, so that high use concentrations result.